My kaiser bariton is going to end up being 5+1 out of necessity (unless I wanted to [1] tear it all back apart, and [2] throw away the long mouthpipe tube and start over), but I would have preferred that it be 4+2.
5+1 insures that no digit is available to move the #1 (if needed), whereas 4+2 (again) leaves a thumb open for a possible #1 trigger as well as another thumb for a possible #5 trigger.
With my particular F tuba tuba, I never need to move the #1 slide, so I've got my right hand thumb operating a (rarely-used) #5 slide trigger...
I barely move it out for 5-6-4 (low A), and - for a nice resonant (rather than the open/"hollow"-sounding) low F - I kick out the #5 slide as far as it will go for a 5-6-1-2-3-4 (24 feet of tubing - just as with a Bb or C tuba) low F...That's my Rite of Spring low F "secret weapon".
Something else positive about the 5-6-1-2-3-4 set-up:
The chromatic fingering pattern actually make logical-to-the-brain sense, as valves 5 and 6 (operated by the left hand's index and middle fingers) mirror the chromatic pattern (learned by all brass-playing beginner players within the first month of playing) in the right hand...except (of course) with the 4th valve depressed.
In contrast, a 5+1 system is an *"add-on" (rather than integrated) configuration.
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*ie. "I'm accustomed to this, but I'm sticking this on here, too."